1. Field of the Invention
The present invention relates to a donor substrate for a laser induced thermal imaging (LITI) method and an organic electroluminescent (EL) display device fabricated using the same and, more particularly, to a donor substrate used for forming an organic layer of an organic electroluminescent display device, and an organic electroluminescent display device fabricated using the same.
2. Description of the Related Art
In general, an organic electroluminescent (EL) display device includes various layers such as an anode, a cathode, a hole injection layer, a hole transport layer, an emission layer, an electron transport layer, an electron injection layer, etc.
The organic EL display device is classified into a polymer device and a small molecular organic EL device according to materials. In the case of the small molecular device, each layer is introduced by a vacuum deposition process. In the case of a polymer organic EL display device, an EL element is fabricated by a spin coating process.
In the case of a mono-color display device, the polymer organic EL display device may be easily fabricated by the spin coating process and driven at relatively low voltage compared to the small molecular organic EL display device, but is lowered in efficiency and lifetime. On the other hand, in the case of a full-color display device, red, green and blue polymers should be patterned, respectively. However, an ink jet method or a laser induced thermal imaging method is not preferable because it deteriorates luminescence characteristics such as efficiency, lifetime, etc. of the organic EL display device.
In particular, most single polymer material is hardly transferred by the laser induced thermal imaging method. Various methods for patterning the polymer organic EL display device using the laser induced thermal imaging method are disclosed in Korean Patent No. 1998-51844, and U.S. Pat. Nos. 5,998,085, 6,214,520, and 6,114,088.
The laser induced thermal imaging method requires at least a light source, a transfer film, and a substrate, wherein light emitted from the light source is absorbed in a light absorption layer of the transfer film and transformed into heat energy, so that the heat energy transfers a material for forming a transfer layer from the transfer film to the substrate, thereby forming a desired image (refer to U.S. Pat. Nos. 5,220,348, 5,256,506, 5,278,023, and 5,308,737).
Such a laser induced thermal imaging method has been used in fabricating a color filter for a liquid crystal display device, or in patterning an EL material (refer to U.S. Pat. No. 5,998,085).
U.S. Pat. No. 5,937,272 relates to a method of forming a highly patterned organic layer in a full-color organic EL display device, in which an organic EL material employs a donor support coated with a transferable material. In this method, the donor support is heated to transfer the organic EL material to a recessed portion of a substrate corresponding to a desired lower pixel, wherein the recessed portion is provided for forming a colored organic EL medium. At this time, the transfer is accomplished by applying heat or light to the donor support to vaporize the organic EL material, thereby transferring it to the pixel.
U.S. Pat. No. 5,688,551 relates to a method of forming a sub-pixel formed in each pixel region, in which the organic EL material is transferred from a donor sheet to a receiver sheet, thereby forming the sub-pixel. At this time, the transfer is performed at a low temperature of about 400° C. or less, and a sublimatable organic EL material is transferred from the donor sheet to the receiver sheet, thereby forming the sub-pixel.